It's 100% the latter. I get "ok we don't want to mix codebases" and that's fine, but if you can code in C# you can probably get up and running in F# in a week, maybe a month if you struggle with some of the concepts.
One major issue I do see coming from the C# side is "well how do I do this then?!", which often the answer is "you don't, because you don't need to" or "well what if it's more performant to do it mutably!" well then thankfully F# can absolutely do that.
If you keep an open mind it's really a very clean and simple language, but in an age where half of development is importing 8 well known libraries, not being the main supported language is a major weakness.
I normally write C++, but I've also written in Rust, Python, Ruby and bunch of other languages. I've never had trouble with a programming language until Haskell, and had to accept that I'm just probably just not smart enough to do it.
However, I wrote my first thing a port of a small C# tool of a couple hundred lines, to F# in about an hour. It's been about a week now, and things are considerably smoothing out.
To be considered niche, the F# tooling has been great. Also, having the .NET libraries available adds a lot of built-in capability.
